Let's begin to explore the Sun's
kingdom, close to home, by looking at the inner, rocky worlds.

Mercury – the Sun-baked Mini-Planet

The closest planet to the Sun is sunbaked Mercury. Much of what we know about
this strange, rocky world came from the spacecraft Mariner 10, which flew by it
three times in 1974 and 1975.

Mercury is a tiny planet, only 3,033 miles (4,880 kilometers) in diameter. It
has no atmosphere and no water. Because of its closeness to the Sun, Mercury gets
extremely hot during the day – as much as 800°F (427°C)! At night,
it cools down to a very chilly -350°F (-212°C). These extremes make Mercury
the planet with the widest temperature range of any in the Solar System.

Mercury's day – the time it takes to spin once around on its axis –
is equal to 59 Earth days. Its year – the time it takes to go once around
the Sun – is equal to 88 Earth days. As a result, Mercury manages to fit
just 3 days into every 2 of its years! During each of these long Mercurian days,
the Sun appears 3 times bigger and 10 times brighter in the sky than it does on
Earth.

Mariner 10's cameras showed that Mercury has a surface very much like
that of the Moon. It is covered by craters of all sizes, and has at least one
great plain – the Caloris Basin – that is surrounded by rings of mountains.
Most of these scars on Mercury's surface were probably caused long ago by meteors
and asteroids that crashed into it. Because there is no weather on Mercury to
wear down the craters and mountains, they have remained more or less unchanged
for billions of years.

Like all the rocky worlds, Mercury is made of a heavy iron core, surrounded by
a mantle and a crust of lighter rocks. Mercury's iron core must be quite large
because the planet is heavy for its small size.

Venus – the Cloud-Covered Oven

Venus, the next planet out from the Sun, was for a long time an even bigger mystery
than Mercury. Although Venus is our closest neighbor in space apart from the Moon,
its surface is always hidden beneath a thick layer of bright, fleecy clouds. Thanks
to a number of space probes, however, we have peeked under the clouds of Venus
and learned some of its secrets.

Venus has been called Earth's "sister" planet. In fact, the two worlds are almost
the same size. Venus is 7,523 miles (12,104 kilometers) in diameter, Earth 7,928
miles (12,756 kilometers). They are probably very much alike inside, too. Still,
these sister planets are different from each other in important ways.

Venus takes longer to spin on its axis than it does to go around the Sun. In other
words, its "day" (lasting 243 Earth days) is longer than its "year" (lasting 225
Earth days). Stranger still, Venus spins backwards. If you could stand on Venus's
surface and see through its thick clouds, you would see the Sun rise in the west
and set in the east more than 3½ of our years later!

In other ways, too, Venus is very different from our planet. Its atmosphere weighs
90 times more than Earth's. Standing on Venus, you would have as much weight over
your head as a diver would half a mile under the ocean!

The main gas in Venus's atmosphere is carbon dioxide. This gas works like the
glass in a greenhouse. It traps heat from the Sun and causes the temperature at
the surface of Venus to become very high. Venus is, in fact, hotter than Mercury,
even though – at 67 million miles (108 million kilometers) – it's
almost twice as far from the Sun. Venus's surface temperature, both day and night,
stays above 850°F (more than 450°C).

Once Earth's atmosphere was probably much like Venus's is today. Billions of years
ago, volcanoes on both planets released large amounts of carbon dioxide into the
atmosphere. But on Earth, most of the carbon dioxide was later taken up by plants.
The plants produced oxygen, which made the life we know today possible.

Now, today, through neglect, the Earth's people are making its atmosphere more
like Venus's again. We are releasing carbon dioxide into the air by burning coal
and oil. At the same time, we are cutting down huge areas of forest that once
turned carbon dioxide into life-giving oxygen. Will we learn to change our ways
before Earth, too, becomes an enormous, hot greenhouse?

Venus certainly has some unwelcome surprises for the space traveler. Not only
is it very hot, with a crushingly heavy atmosphere, but its surface seems to be
a barren wasteland. Pictures sent back from the surface show a gloomy desert of
tumbled rocks. Radar measurements, taken from above the clouds, show wider areas
of lifeless craters, valleys, and plains.

Lightning, too, is common on Venus. Lightning strikes on the ground and the steady
glow of lightning in the sky will greet any future visitors. Higher up, the yellowish
cloud tops are filled with sulfuric acid, a dangerous chemical. To travel safely
through the burning acid and lightning, spacecraft need to be well protected.

Venus would be the last place in the Solar System to visit for a holiday!

Earth – the Planet of Life

Perhaps, instead, we should stay home on Earth – the third planet from the
Sun. It is the most beautiful and comfortable planet in the Sun's kingdom. More
importantly, it is the only world on which we are certain that there is life.

At a distance of 93 million miles (150 million kilometers), Earth receives just
the right amount of heat and light from the Sun. Unlike Mercury or Venus, it doesn't
bake. And unlike the outer planets, it isn't a frozen ball of icy gases and rock.
Apart from its hot deserts and chilly poles, Earth is a good place for living
things.

How did life first develop on our planet? About 4 billion years ago, Earth probably
had a choking atmosphere of carbon dioxide and other unpleasant gases. Its surface
was bombarded by deadly ultraviolet rays from the Sun. But then a wonderful thing
happened.

Because the temperature was just right, water vapor in the Earth's early atmosphere
formed thick clouds. Soon it began to rain everywhere. It rained so hard, and
for so long, that huge oceans were formed.

Over millions of years, these oceans became the birthplace of the first living
things on Earth. Tiny plants appeared, shielded by a thin layer of sea water from
the Sun's killing ultraviolet rays.

Slowly, the first plants turned much of the carbon dioxide in the atmosphere into
oxygen. This is the gas that humans, and all other animals, breathe. At the same
time, high in the atmosphere, ultraviolet rays turned some of the oxygen into
ozone. That change was important because ozone blocks out ultraviolet. A layer
of ozone was formed around the Earth that protected all life on its surface from
the Sun's harmful rays. In the millions of years that followed, life spread out
from the oceans to the land and developed into the countless forms we know today.

Mars – the Red World of Mystery

Only one other planet in the Solar System seems to be anything like Earth. This
is the Red Planet, Mars, the outermost of the rocky worlds. We have learned a
great deal about Mars from space probes that have been sent to look at it close
up. In 1976, Viking 1 and 2 became the first spacecraft to carry
instruments onto its surface.

Mars is a smaller planet than Earth, just 4,218 miles (6,787 kilometers) in diameter.
At a distance of 142 million miles (228 million kilometers) from the Sun, it is
also quite a bit cooler. Mars has two, tiny, potato-shaped moons – Phobos
and Deimos – that are probably captured asteroids.

On the Martian surface, the cameras of spacecraft have shown an amazing mixture
of craters and plains, giant canyons, and monstrous volcanoes. Mars's biggest
canyon – Valles Marineris – dwarfs our own Grand Canyon in
Arizona. Its greatest volcano – Olympus Mons – towers three times
higher than Mount Everest.

Most unexpected of all, the Mariner 9 and Viking probes found
dried-up river beds on Mars. These show that the Red Planet was once a lot wetter
than it is today. Scientists think that most of Mars's water is now locked up
in layers of frost deep underground. In the past, though, it may have been brought
to the surface during periods of great volcanic eruptions.

Did life develop in the waters of Mars long ago, as it did on Earth? If so, are
there living things on Mars today?

We still don't know for sure. The Viking landers found nothing definite in their
search for life, and the Red Planet as a whole seems to be an unfriendly place.
Its surface – reddened by iron oxide (rust) – is bone dry. Its atmosphere
of carbon dioxide is 100 times thinner than Earth's. Its weather sometimes produces
130-mile-per-hour winds that blow dust storms over most of the planet. And its
temperatures are usually well below zero.

On the other hand, it's possible that simple living things – germs, for
instance – might survive in certain places on Mars. Frozen surface water
has been found in the Martian polar caps. There may even be water droplets under
the soil of the deeper valleys. And in summer, parts of Mars can warm up to 80°F
(26°C).

Perhaps to settle the question of life on Mars – and other mysteries of
the red Planet – we shall have to go there ourselves. Mars may well be the
next target for the astronauts and cosmonauts of our world.